How to assign an object property value to a variable in Java?

Question

The following code is what I'm having issues understanding:

public class Rectangle {

    public Rectangle() {
        double width = 1;
        double height = 1;
    }

    public Rectangle(double w, double h) {
        double width = w;
        double height = h;
    }

    public double getArea(double w, double h) {
        return (w*h);
    }

    public double getPerimeter(double w, double h) {
        return ((2*w)+(2*h));
    }

    public static void main(String[] args) {
        Rectangle oldRectangle = new Rectangle(4, 40);
        Rectangle newRectangle = new Rectangle(3.5, 35.9);
        double height1 = oldRectangle.height;
        double height2 = newRectangle.height;
        double width1 = oldRectangle.width;
        double width2 = newRectangle.width;

        System.out.println("Width of Rectangle 1 is: " + 4);
        System.out.println("Height of Rectangle 1 is: " + 40);
        System.out.println("Area of Rectangle 1 is: " + oldRectangle.getArea(4, 40));
        System.out.println("Perimeter of Rectangle 1 is: " + oldRectangle.getPerimeter(4, 40));
        System.out.println("Width of Rectangle 1 is: " + 3.5);
        System.out.println("Height of Rectangle 1 is: " + 35.9);
        System.out.println("Area of Rectangle 1 is: " + newRectangle.getArea(3.5, 35.9));
        System.out.println("Perimeter of Rectangle 1 is: " + newRectangle.getPerimeter(3.5, 35.9));

    }

}

---

I was instructed to create two constructors for a Rectangle class—one with no arguments but assigned a default value of 1 for both variables width and height. The second constructor was to contain parameters that would take in two doubles that would get assigned to their appropriate variable.

I was then told to create two 'get()' methods that returned their respective values—in my case, they were perimeter and area of said Rectangle.

I was then instructed to create two Rectangle instances, one with a width of 4 and a height of 40—and another with a width of 4.5 and a height of 35.9. So, I did just that and made two new Rectangle objects, as you can see.

Lastly, I was instructed to print out the Width, Height, Perimeter, and Area of both Rectangle objects. My issue is that I don't know of a way to reference them. I took a beginners tutorial class for JavaScript and if I'm not mistaken, I recall there was a way to reference a property value of an object by assigning it to a variable. Again, I'm saying "If I'm not mistaken", so I could be wrong. It's been a while...

I do realize that Java and Java Script are entirely different things in their own right. Java Script was a scripting language developed and based off of Java.

Anyway, any help will be grand.

Please feel free to help me understand how I can implement what I'm trying to do by giving examples. You don't have use my exact code, but I'd like to be able to get my code to make more sense.. I'm using Eclipse btw.

Answer 1

You are on the right track. What you are looking to do is something like this:

public double getArea(Rectangle r){
      return r.width*r.height;
}
public double getPerimeter(Rectangle r){
      return (2*r.width + 2*r.height);
}

For the print statements you are hard-coding in values which you dont have to do.

 System.out.println("Width of Rectangle 1 is: " + oldRectangle.width);
    System.out.println("Height of Rectangle 1 is: " + oldRectangle.height);
    System.out.println("Area of Rectangle 1 is: " + getArea(oldRectangle));
    System.out.println("Perimeter of Rectangle 1 is: " + getPerimeter(oldRectangle));

Answer 2

My issue is that I don't know of a way to reference them

As Java is a strong-typed language, to store the references to each Rectangle (or any object) attribute, you must create variables that match the field.

First, you need your rectangle have attributes:

public class Rectangle {
    public double width;
    public dobule height;

    // constructors here...

After, methods to calculate area and perimeter:

public double getArea() {
    return (width*height);
}

public double getPerimeter() {
    return ((2*width)+(2*height));
}

Now, to reference them (i guess you still mean area and perimeter):

double area1 = getArea(newRectangle);
double area2 = getArea(oldRectangle);

and

double perimeter1 = getPerimeter(newRectangle);
double perimeter2 = getPerimeter(oldRectangle);

Answer 3

There are no width and height properties in your class, they are only defined inside the constructor, declare them in the class and not inside the constructors so you can acces them like this:

public class Rectangle {

    double width;
    double height;

  public Rectangle() {
      width = 1;
      height = 1;
  }

public Rectangle(double w, double h) {
    width = w;
    height = h;
}

public double getArea(double w, double h) {
    return (w*h);
}

public double getPerimeter(double w, double h) {
    return ((2*w)+(2*h));
}
//....
}

Note: It's better to declare them as private and use getters and setters to acces them, take a look at Adding Setter and Getter Methods for further information.

EDIT:

In your case you don't need to pass parameters to your methods (because the calculation needs this rectangle width and height), just calculate them using your class fields, like this:

public double getArea() {
    return (width*height);
}

public double getPerimeter() {
    return ((2*width)+(2*height));
}

Here's what you need to do to acces your variables:

public static void main(String[] args) {
    Rectangle oldRectangle = new Rectangle(4, 40);
    Rectangle newRectangle = new Rectangle(3.5, 35.9);

    //In the following you access all the object variables and methods

    System.out.println("Width of Rectangle 1 is: " + oldRectangle.width);
    System.out.println("Height of Rectangle 1 is: " + oldRectangle.height);
    System.out.println("Area of Rectangle 1 is: " + oldRectangle.getArea());
    System.out.println("Perimeter of Rectangle 1 is: " + oldRectangle.getPerimeter());
    System.out.println("Width of Rectangle 1 is: " + newRectangle.width);
    System.out.println("Height of Rectangle 1 is: " + newRectangle.height);
    System.out.println("Area of Rectangle 1 is: " + newRectangle.getArea());
    System.out.println("Perimeter of Rectangle 1 is: " + newRectangle.getPerimeter());

}

You can test the DEMO here.

And to answer your question about using constructor, calling the parametrized constructor like this:

Rectangle oldRectangle = new Rectangle(4, 40);

Is equivalent to:

// create the object
Rectangle oldRectangle = new Rectangle(); 
//And then assigning the values 4 and 40 to its width and height    
oldRectangle.width=4; 
oldRectangle.height=40;